Selective hardening of age-hardenable alloys and articles produced thereby



United States Patent C) 3,355,333 SELECTIVE HARDENING OF AGE-HARDEN- ABLE ALLOYS AND ARTECLES PRODUCED THEREBY Alan George Haynes, Sevenoaks, Kent, and George Mayer, Colohann, Surrey, England, assignors to The International Nickel (Zompany, Inc., New York, N.Y., a corporation of Deiaware No Drawing. Filed Sept. 22, 1964, Set. N 398,414 Claims priority, application Great Britain, Sept. 24, 1963, 37,540/63 8 Claims. (Cl. 148-142) The present invention relates to the hardening of articles made from age-hardenable alloys and, more particularly, to the selective hardening of portions of articles made from age-hardenable alloys.

Heretofore, the art has endeavored to harden areas or zones of articles or parts in order to improve their performance under heavy stress and to give added resistance to deleterious phenomena such as pitting, fatigue, indentation, wear, erosion, corrosion and other forms of damage. It is often impracticable or undesirable to give the necessary resistance to such forms of failure by making the whole article or part from a homogeneous mass of hardenable alloy and thereafter hardening it throughout its cross section to develop the properties of hardness and strength. This can be due to many considerations. In particular, the necessary high hardness and tensile strength or compressive strength is often not obtained without sacrifice of ductility, loss of resistance to fracture when subjected to high rates of stressing, and reduced strength in the presence of sharp changes in section such as notches. This sacrifice throughout an article or part would render it more susceptible to failure under some conditions of stress. The production of an entire article or part from a homogeneous mass of an alloy of high hardness also has the disadvantage of making subsequent fabrication such as forming or machining more difiicult or impossible.

In order to overcome the disadvantages which result from hardening a part throughout its cross section, the art has turned to the use of case hardening processes. These processes which include carburizing, cyaniding, carbonitriding, nitriding, induction hardening and flame hardening result in a product having a hardened surface or portion thereof known as the case. The remainder of the product is retained in an unhardened, therefore more ductile and more easily machinable, condition than the case, and this unhardened portion is commonly known as the core. While these processes have been satisfactorily appiled to, for example, conventional lowor medi um-carbon, low-alloy steels, difliculties are encountered when attempts are made to apply them to age-hardenable alloys, e.g. age-hardenable steels. The carbon introduced into an alloy during carburizing, cyaniding and carbonitriding treatments tends to combine with the precipitable elements in an age-hardenable alloy, therefore drastically reducing the hardness attainable upon aging. Induction or flame hardening processes which comprise heating the surface of a product to a high temperature and quench ing to harden would result in a very soft or solution heat treated surface condition when applied to age-hardenable alloys. Nitriding treatments are generally unsatisfactory 3,355,333 Patented Nov. 28, 1967 due to the excessive time required to produce a given depth of case. For example, 50 to hours are required to produce a maximum case depth of from 0.02 to 0.03 inch. Moreover, when applied to age-hardenable materials, the high temperatures (500 C. to 650 C.) and long heating periods required by the nitriding treatment can be detrimental to the mechanical properties of the core.

Although attempts were made to overcome the foregoing ditficulties and other disadvantages, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that selected portions of an article or part made from an age-hardenable alloy can be hardened by a special aging treatment.

It is an object of the present invention to provide a process for the selective hardening of portions of an article made from a homogeneous mass of an age-hardenable alloy.

Another object of the invention is to provide a process for the selective hardening of portions of an article made from a homogeneous mass of an iron-base age-hardenable alloy.

The invention also contemplates providing a process for the selective hardening of portions of an article made from a homogeneous mass of maraging steel.

It is a further object of the invention to provide an article formed from a homogeneous mass of an agehardenable alloy having a harder age-hardened portion and a softer portion.

Other objects and advantages will 'become apparent from the following description.

Broadly started, the present invention relates to agehardenable alloys, that is to say, alloys in which a precipitable phase dissolves in a matrix when the alloy is heated to an elevated temperature and is retained in solution when the alloy is thereupon cooled at an appropriate rate, these steps comprising a solution heat treatment, and in which the precipitable phase is precipitated when the alloy is reheated to a temperature lower than that of the solution heating thereby hardening the alloy relative to the solution-heated and cooled state. This precipitation step is commonly known as aging, and the extent of the aging is a function of time and temperature. In general, the initial precipitates, a term which includes the so-called Guinier-Preston zones, are extremely finely dispersed, and the hardness increases with the amount of such finely dispersed precipitates that is formed. In the course of time the fine precipitates tend to agglomerate, this tendency being increasingly pronounced as the aging temperature increases, and the agglomerates do not impart the same hardness as the finely dispersed precipitates. If the aging is continued for too long, particularly at a fairly high temperature, the precipitate tends not only to agglomerate but also to redissolve and the alloy softens further. With any given reheating temperature there is thus a stage at which the hardness is at a maximum and the alloy is then said to be fully aged. Below this stage the alloy is said to be un-deraged and when this stage is passed, the alloy is said to be overaged. It will be understood that for any given alloy the maximum hardness obtainable and the time required to reach it will depend on the hardening temperature employed.

a According to the invention an article or part of an agehardenable alloy is solution-heated, cooled to preserve the solution thus formed, and thereafter one zone is rendered harder and stronger than another by localized aging. There are thus harder and softer zones in the treated article and part. Normally, it is desired to harden a surface portion of an article, but if a particular article requires a relatively soft surface on a relatively hard base, or it is desirable to harden one portion of an article throughout its cross section while maintaining the re mainder of the article in a soft condition, then appropriate diflerential aging may be effected.

I Differential or selective age hardening, i.e., aging of the selected zone without aging of the entire component to a similar hardness, depends upon the setting up of thermal gradients and also upon the rate at which the age-hardening process proceeds at the aging temperatures attained during selective hardening. It can be ac complished by locally heating the selected zone at a high rate, for example, by immersion in a hot fluidized bed or molten salt or metal bath, by flame heating, or by electrical induction or resistance heating. The necessary thermal gradients can be :set up by the use of high rates of local heating so that the remainder of the article or part remains cold, and, if desired, the whole article or part may be cooled to a low temperature before heating begins. Advantageously, the zone that is not to be hardened may be chilled during the heating. For example, a tubularartiole such as a gun barrel can be cooled externally in water and heated internally, the reverse procedure being used when the external surface must be hard.

It may not always be possible in practice to sustain the temperature gradient or selective heating conditions for prolonged periods, and the heating conditions must be determined with proper regard to the composition of the alloy. Alternatively, if the article or part in question can be made from any one of a number of alloys, then the alloy may be so chosen that the desired change of hardness or strength is attained under specific heating conditions. It may be desired to choose an alloy which is only partly aged under these conditions, and which on being fully aged by more prolonged heating would reach a much greater hardness.

In carrying the invention into practice, it is preferred to use the selective hardening treatments of the present invention on iron-base age-hardenable alloys with a territic, martensitic, austenitic or bainitic type matrix structure as well as on nickel-base age-hardenable alloys.

The invention is particularly applicable to articles or obtained, the composition of the alloy should, advanta-' geously, be such that when aged at 700 C., it will attain a hardness of at least 300 DPN (diamond pyramid hardness number), and preferably, at least 400 DPN, in the course of 100 hours.

The invention is most particularly applicable to alloys which can be age hardened in the martensitic condition to the hardness desired. In this specification the term martensitic is used to describe those alloys which have or can be caused to have a matrix structure composed substantially of martensite and the term martensite includes low temperature transformation products of austenite.

Alloys that can be age hardened in the martensitic state include the recently developed maraging steels. In one family of such steels the precipitable phase isbased on titanium with or Without aluminum. These steels contain from 18% to 30% nickel and from 1.5% to 9% e.g.,

1.5% to 3% or 5%, in all of titanium or aluminum or both with or without other elements. Some ofthese steels are described and claimed in US. Patent No. 3,093,518. When such steels contain from 18% to 24% nickel, they become martensitic on cooling from a high temperature. of 760 C. or above to room temperature or below, and then can be aged by heating for A 'hour to 24 hours at 260 C. to 650 C., preferably not above 540 C. It is advantageous to use these steels containing no more than 24% nickel because of the ease with which they may thus be hardened.

In the family of steels just discussed, those containing from 24% to 30% nickel shall 'be referred to for purposes of this invention as being austenitic-martensitic. They can be hardened by solution heat treating, aging for from 1 hour to 24 hours at a temperature above the martensitic transformation range and within the range 590 C. to 760 C., cooling to below 32 C. to produce a martensitic structure and then aging for from hour to 24 hours in the temperature range 260 C. to 650 C.

Preferably, the temperature in the second aging step does not exceed 540 C. These steels can be hardened to very high values and before being harneded can readily be machined. It is advantageous, therefore, to use them when very high final hardness is required, even though the heat treatment is more complex. 9

Another family of steels that can be aged in the mar-- tensitic state (maragiug steels) depends primarily on molybdenum and cobalt for the hardening, 'though'additiona-l hardening may be imparted by one or moreof other elements, namely, carbon, silicon, titanium, aluminum, copper, tungsten, niobium, vanadium, beryllium and nitrogen. These steels contain from 10% to 23% nickel, from 1% to 10% molybdenum and from 2% to I 30% cobalt with or without other elements and are described in detail in US. Patent No. 3,093,519. The hardness of these steels after aging depends upon the content of the hardening elements, i.e., those that enter the composition of the phases precipitated on aging and particularly upon the contents of molybdenum and c0- balt, the product of the numerical values of the percentages of which should be from 10 to 100. The steels may 7 also contain various other elements. Such steels develop high strength when heated in the martensitic state for 3 hours at 480 C. The level of ductility associated with the high strength and hardness is dependent upon com-v position. An article or part of such a steel may be heated to a lower temperature, say, between 300 C. and 400 C. for 3 hours to develop moderate hardness and then the zone to be hardened may be fully aged by rapid heating, say, to 550 C. for 5 minutes.

If effective use of the invention is to be made, the alloy should be rich in the precipitable elements since if it is not, no great advantage, i.e., difference in properties in the different zones, will be gained by means. of the invention. Broadly, it may be said that any alloy which is hardened in the martensitic state should advan tageously (if it is to be used in the invention) have a composition such that if it is aged at 480 C., it will attain a hardness of at least 450 DPN in the course of hours. It is even better if it attains a hardness of 600 DPN and better still if it attains a hardness of 700 DPN in the course of 100 hours when so aged.

The differential or selective hardening process of the present invention can be effected in various ways, as indicated in Tables -I and II. The methods listed in these tables are the more simple operations most likely to be of practical use, but others will readily be apparent.

Table I deals with the treatments which are applicable to iron-base age-hardenable alloys with a ferritic, mar tensitic, austenitic or 'bainitic type matrix structure, to

nickel-base age-hardenable alloys and to other agehardenable alloys in which precipitation does not promote a gross phase change of the-matrix.

TABLE I Treatment Method Zone 1st 2nd 3rd Stage 4th Stage Stage Stage A Harder" Soln U.A Selective RA--.

Softer..- Soln U.A K.C

B Harden- Soln K.C

Soiter Soln Selective- O.A C Harden- Soln F Softer"- Soln F D Harder" Sol'n Selective- F.A Softer". Sol-nun K.C E Harder" Soln O.A Selective Soln-. FA. Softer.-- Soln O.A K.C RA.

F Harden- Soln U.A Selective Soln Selective RA.

Softer Soln U.A K.C K.C.

Soln= Solution heat treated. F. Subjected to full aging treatment. 0.- =Overaged to lower hardness. U.l Underaged to lower hardness. K.C.=Kept colder than the temperature previously attained by this zone of the article or part, and kept below the temperature of the selectively heated zone.

Preferably, the Whole article or part is first underaged TABLE H to give a good combination of strength and ductility and the zone which should be hard is further aged to a higher Treatment hardness (Table I, Method A). Normally this further thod Zone aging will involve substantially or completely full aging, 1st Stage 2nd stage 3rd Stage but as explained above, it may be desirable to use an alloy that gives the desired hardness in a partially aged stage. AA Harder" s 1 A1 Selective AZ Another way of effecting the differential aging com- VBB g g g ljnml kl u u flu go A prises overaging the zone which is required to be softer 8 3:: we while keeping cool the zone which must be harder and CC Harder Softer-.. Soln KC A2. then further aging the entire article or part to bring the selected zone to high hardness and strength (Table I, Method B). This step of further aging has little or no effect on a zone that has already been overaged. This method is not as satisfactory as the preferred method (Method A above) because the ductility of the overaged zone is not as high as that of a zone which has only been partly aged. Similar comments apply to the Method C in Table I, which is essentially a reversal of the aging treatment sequence just described. In this procedure, the entire article or part is aged to the hardness desired for the harder zone; the zone which is required to be softer is then selectively overaged While the harder zone is kept cool.

Method D of Table I involves selective heating of the zone required to be harder to develop the desired harrness. The properties developed in the softer zone will be dependent on the temperature attained as a result of the temperature gradient generated during selective heating of the harder zone. Therefore, the properties of the softer zone are subject to less control than in Methods A, B and C of Table I.

A further method (Table 1, Method B) comprises overaging the whole article or part to give the desired lower hardness combined with reasonable ductility and then selectively solution heat treating the zone which must be hard. Thereafter, the whole article or part is subjected to a further aging treatment to give high hardness in the zone which was solution heat treated.

Another way of selective age hardening which has some similarity to the previous method is described as Method F, Table I. The whole article or part is underaged to give the desired low hardness and desired combination of strength and ductility; then the zone which must be hard is selectively solution heat treated and then selectively aged, while the softer zone is kept at a temperature below that employed in the initial underaging treatment.

Table II deals with iron-base austenitic-martensitic alloys in which age hardening of the austenite causes a microstructural phase change to martensite which, in turn, can be further age hardened.

A1=First age-hardening treatment. Applied to material in the essentially austenitic condition.

A2= Second age-hardening treatment. Usually applied with effect to mater al previously treated to give a marteusitic or martensitic-austenitic condition.

K.C.=As Table I. Soln=As in Table I.

The normal method of hardening solution heat treated iron-base alloys of the austenitic-nartensitic type involves an initial age-hardening treatment at a comparatively high temperature but below that of solution heat treatment. This treatment results in the formation of a precipitate which in itself will harden the alloy, but in so forming, the precipitate denudes the matrix of alloying elements. The denuded matrix can no longer exist as austenite on cooling to low temperature and thus martensite is formed. This mart-ensite matrix is capable of further age hardening at a lower temperature, thus supplementing the age hardening produced in the original austenite.

elective hardening of austenitic-martensitic alloys can thus be applied in various ways: (a) solely to the age hardening of austenite; (b) solely to the age-hardening of martensite after normal aging and transformation of austenite throughout the entire article or part; or (c) it can be applied to both stages of age hardening.

Method AA of Table II utilizes a uniform age-hardening heat treatment of the austenite throughout the entire article or part. The martensite formed following this treatment is then selectively hardened by utilizing the second and subsequent treatments of any of the Methods A, B, C or D in Table I. The heat treatment cycle used to age the austenite can either be one that will give maximum hardness by full or substantially full aging, or one that will produce a lower hardness, eg, by overaging.

Method BB of Table II employs selective age hardening of the austenite in the zone required to be harder. The zone required to be softer is kept at a lower temperature during this operation and thus will age harden to a smaller extent, if at all, and will not necessarily transform completely or in part to martensite when cooled to roomv temperature or slightly below.

If the desired softer zone does transform substantially or completely to martensite after the second treatment of Method BB, Table 11, then the desired harder zone, which is essentially ma-rtensitic, should be selectively age hardened by utilizing the second and subsequent treatments of any of the methods A, B, C or D in Table I.

If the zone required to be softer remains substantially austenitic after selective age hardening by the second treatment of Method CC, Table II, then the entire component can be subjected to the lower temperature aging treatment. This treatment will produce an age hardened martensite of high hardness in the zone required to be hard but will have little eifect on the desired softer zone.

In every case the precipitable phase must initially be in solution and it will be noted that the first treatment in all methods described in Tables I and II is a solution heat treatment. After annealing, normalizing or but working, however, a separate solution treatment will often be unnecessary. Furthermore, other treatments in the sequence may also be omitted, provided the material has previously been subjected to equivalents of the earlier treatments as a result of other processing.

For the purpose of giving those skilled in the art a better understanding of the invention, a series of maraging steels were selected having the compositions set forth in Table III:

TABLE III.CHEMICAL COMPOSITIONS OF STEELS USED FOR SELECTIVE AGE-HARDENING TESTS Nominal value.

Samples of these steels were given a conventional agehardening treatment comprising a solution heat treatment (1 hour at 820 C.) followed by aging (3 hours at 480 C.). Diamond pyramid hardness readings (DPN) taken on these samples after solution heat treatment and after aging are set forth in Table IV:

TABLE IV Hardness (DPN) Steel No.

Solution Heat Solution Heat Treated 1 hour Treated and Aged,

at 820 C. 3 hours at 480 C.

The heat treatment cycle used in obtaining the data set forth in Table IV, however, is of relatively long duration and the temperatures employed are relatively low in comparison to the localized high temperature, short-duration cycles which result from the selective hardening treatments of the present invention,.e.g., those cycles resulting from immersion in a hot fluidized bed, molten salt or metal bath, or from flame, electrical induction or resistance heating. For example, in selective age hardening by the method set forth in Table 1, Method A, the zone of the article which is to be hardened is brought to the highest ing it amenable to machining and less susceptible to brittle possible hardness by means of a short-time aging cycle which will not affect the hardness of the rest of the article. The data set forth in Table V show the hardnesses developed in 0.03 inch thick samples of the steels set forth in Table III which have been aged for 30 seconds at 600? C. to simulate the rapid local heating of an article during a selective hardening treatment such as Method A of Table I.

TABLE V Hardness (DPN) Steel N 0.

Solution Heat Treated and Aged 30 seconds at 600 C.

It can be seen from a comparison of the data presented in Table V with those of Table IV that the short-time aging cycles contemplated by thepresent invention do; result in a distinct increase in hardness from the solution heat treated condition.

A'further series of samples of the steels set forth in Table HI was given a short-time solution heat treatment (30 seconds at 950 C.) followed by a conventional aging treatment (3 hours at 480 C.) to simulate selective solution heat treatment and aging cycles such as those obtaining in Method E, Table I. The hardness levels attained in these samples are set forth in Table VI: TABLE VI Hardness (DPN) Steel No.

Solution Heat Treated 30 seconds at 950 C. and Aged aged or overaged condition or even in the solution heat treated condition. This softer portion is characterized by a significantly lower hardness and correspondingly greater ductility than the'selectively age-hardened portion, renderfracture. A series of samples of the steels set forthin Table III was placed in an underaged condition, i.e., that ness readings andimpact values obtained for these sam- 9 ples, and set forth in Table VII, indicate that acceptable levels of strength and ductility are retained by these steels in the underaged condition.

Samples of the steels set forth in Table III were treated under conditions such as those obtaining in the unhardened portion of an article when treated, for example, by Method E, Table I. This was accomplished by solution heat treating the samples, overaging the samples by heating for 1 hour at 650 C. and then subjecting the overaged samples to a conventional aging treatment (3 hours at 480 C.). Hardness readings taken on the samples after the overaging treatment and also after the conventional aging treatment are set forth in Table VIII, together with impact values of the samples obtained after the latter treatment.

It will be noted that in comparison with the data set forth in Table V 1, the hardness levels of these steels in the overaged condition are significantly lower than the hardness levels obtained by selective hardening and since ductility of these materials decreases with increased hardness, the overaged material can be expected to be more ductile, more readily machinable and less susceptible to brittle fracture than the same material in an aged-hardened condition. The ductility of steels 7 to 10 in the overaged condition can be improved by lowering their hardness for the reasons set forth above. This can be accomplished by employing a modified seelctive hardening treatment, such as Method E, Table 1. Changing the fourth stage treatment of Method E from a conventional aging of the entire article to a selective aging of the harder zone while the softer zone is maintained in a colder condition will result in a softer zone corresponding in hardness to the values set forth in the second column of Table VIII rather than those set forth in the third column of Table VIII.

The present invention is particularly applicable to the selective hardening of portions of gears, metal forming and working equipment and gun barrels as Well as to the surface hardening of armor plate.

It is to be noted that the present invention is not to be confused with heat treatment processes used to alter properties of age-hardenable alloys other than hardness, e.g., grain size.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to Without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and vairations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for the selective hardening of an article formed from a homogeneous mass of age-hardenable iron-base alloy containing about 18% to 24% nickel and 1.5% to 9% of metal selected from the group consisting of aluminum and titanium to thereby produce a first relatively hard portion and a second relatively soft and ductile portion in said article, comprising solution heat treating said article, cooling the article to retain the precipitable phase of the age hardening alloy in solution, heating said atricle to a first temperature for a time suificient to place said alloy in a relatively soft and ductile underaged condition and thereafter heating only said first portion of said article to a second temperature higher than said first temperature for a time suflicient to age harden said alloy while maintaining said second portion of said article below said first temperature.

2. A process for the selective hardening of an article formed from a homogeneous mass of age-hardenable iron-base alloy containing about 10% to 23% nickel, about 1% to 10% molybdenum and about 2% to 30% cobalt to thereby produce a first relatively hard portion and a second relatively soft and ductile portion in said article, comprising solution heat treating said article, cooling the article to retain the precipitable phase of the age hardening alloy in solution, heating said article to a first temperature for a time sufiicient to place said alloy in a relatively soft and ductile underaged condition and thereafter heating only said first portion of said article to a second temperature higher than said first temperature for a time sufficient to age harden said alloy while maintaining said second portion of said article below said first temperature.

3. A process for the selective hardening of an article formed from a homogeneous mass of an age-hardenable iron-base alloy containing about 18% to 24% nickel and 1.5 to 9% of metal selected form the group consisting of aluminum and titanium to thereby produce a first relatively hard portion and a second relatively soft and ductile portion in said article, comprising solution heat treating said atricle, cooling the article to retain the precipitable phase of the age hardening alloy in solution, heating said article to a first temperature for a time sufficient to place said alloy in a relatively soft and ductile overaged condition, solution heat treating only said first portion of said article while maintaining said second portion of said article below said first temperature and thereafter heating said article to a second temperature below said first temperature for a time suflicient to age harden said first portion.

4. A process for the selective hardening of an article formed from a homogeneous mass of an age-hardenable iron-base alloy containing about 10% to 23% nickel, 1% to 10% molybdenum and 2% to 30% cobalt to thereby produce a first relatively hard portion and a second relatively soft and ductile portion in said article, compris ing solution heat treating said article, cooling the article to retain the precipitable phase of the age hardening alloy in solution, heating said article to a first temperature for a time suflicient to place said alloy in a relatively soft and ductile overaged condition, solution heat treating only said first portion of said article while maintaining said second portion of said article below said first temperature and thereafter heating said article to a second temperature below said first temperature for a time sulficient to age harden said first portion.

5. A process for the selective hardening of an article 1 1 formed from a homogeneous mass of age-hardenable, austenitic-martensitic, iron-base alloy to thereby produce a first relatively hard portion and'a second relatively soft and ductile portion in said article, said alloy containing about 24% to 30% nickel and about 1.5% to 9% of metal selected from the group consisting of aluminum and titanium, comprising solution heat treating said article, cooling said alloy whereby the alloy is substantially austenitic, heating said article at a temperature above the martensitic transformation range and Within the temperature range of about 590 C. to about 760 C. for a time sufficient to age harden said alloy in the austenitic state, cooling said article to a temperature below about 32 C.

'to thereby transform said alloy to mortensite and there- 7 after heating only said first portion of said article to a second temperature within the range of about 260 C. to

650 C. for a time sufficient to age harden said alloy in the martensitic state while maintaining said second portion of said article below said first and second tempera.-

tures.

6. A process for the selective hardening of an article formed from a homogeneous mass of age-hardenable, austenitic-martensitic, iron-base alloy to thereby produce a first relatively hard portion and a second relatively soft and ductile portion in said article, said alloy containing about 24% to 30% nickel and about 1.5% to 9% of metal selected from the group consisting of aluminum and titanium, comprising solution heat treating said article, cooling said alloy whereby the alloy is substantially austenitic, heating only. said first portion of said article at a temper- I ature above the martensitic transformation range and within the temperature range of about 590 C. to about 760 C. for a time sufiicient to age harden said alloy in the austenitic state while maintaining said second portion of said article below said first temperature, cooling said first portion to a temperature below about 32 C. to there by transform same to martensite, and thereafter heating said first portion of said article to a second temperature within the range of about 260 C. to 650 C. for a time sufiicient to age harden said alloy in the martensitic state while maintaining said second portion of said article below said first and second temperatures.-

above the rna'rtensitic transformation range and within,

the temperature range of about 590 C. to about 760 C. for a time suflicient to age harden said alloy in the austenitic state while maintaining said second portion below said first temperature, cooling said first portionto a temperature below about 32 C. to thereby transform same to martensite,and thereafter heating said article to a second temperature within the range of about 260 C. to 650 C. for a time sufiicient to age harden said first portion in the martensitic'state. 4 r

8. An article formed from a homogeneous mass of a steel containing about 24% to about 30% nickel and about 1.5% to about 9% of metal selected from the group consisting of aluminum and titanium, said article having a first relatively hard portion and a second relatively soft and ductile portion, whereby said first portion has an agehardened martensitiestructure and said second portion has an age-hardened austenitic structure.

References Cited UNITED STATES PATENTS 2,397,997 .4/1946 Wyche a a1. 14s 1 2x 2,908,565 10/1959 Nelson Mix-142x 3,093,518 6/1963 Bieber --124X 3,093,519 6/1963 Decker et al. 75 124x FOREIGN PATENTS 744,144 2/1956 Great Britain.

CHARLES N. LOVELL, Primary Examiner. 

5. A PROCESS FOR THE SELECTIVE HARDENING OF AN ARTICLE FORMED FROM A HOMOGENOUS MASS OF AGE-HARDENABLE, AUSTENITIC-MARTENSITIC, IRON-BASE ALLOY TO THEREBY PRODUCE A FIRST RELATIVELY HARD PORTION AND A SECOND RELATIVELY SOFT AND DUCTILE PORTION IN SAID ARTICLE, SAID ALLOY CONTAINING ABOUT 24% TO 30% NICKEL AND ABOUT 1.5% TO 9% OF METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND TITANIUM, COMPRISING SOLUTION HEAT TREATING SAID ARTICLE, COOLING SAID ALLOY WHEREBY THE ALLOY IS SUBSTANTIALLY AUSTENITIC, HEATING SAID ARTICLE AT A TEMPERATURE ABOVE THE MARTENSITIC TRANSFORMATION RANGE AND WITHIN THE TEMPERATURE RANGE OF ABOUT 590*C. TO ABOUT 760*C. FOR A TIME SUFFICIENT TO AGE HARDEN SAID ALLOY IN THE AUSTENITIC STATE, COOLING SAID ARTICLE TO A TEMPERATURE BELOW ABOUT 32*C. TO THEREBY TRANSFORM SAID ALLOY TO MORTENSITE AND THEREAFTER HEATING ONLY SAID FIRST PORTION OF SAID ARTICLE TO A SECOND TEMPERATURE WITHIN THE RANGE OF ABOUT 260*C. TO 650*C. FOR A TIME SUFFICIENT TO AGE HARDEN SAID ALLOY IN THE MARTENSITIC STATE WHILE MAINTAINING SAID SECOND PORTION OF SAID ARTICLE BELOW SIAD FIRST AND SECOND TEMPERATURES. 